The present invention relates to novel compounds endowed with sequestering activity with regard to alkaline-earth metals, thus useful in industrial water treatment as additives for detergent compositions.
The invention also relates to a process for the preparation of said compounds.
In the recent years great importance has been given to the substitution of products used in the industrial water treatment field and to the substitution of detergency additives with less toxic and more ecologically consistent products. For this reason, substitutes for products such as tripolyphosphates, phosphonates and polyacrylates, have been studied broadly. All these products are very good alkaline-earth metal sequestering agents, but they also present some considerable drawbacks: tripolyphosphates and phosphonates are very difficult to eliminate from waste waters, with danger of pollution of water-bearing strata and eutrophization. Polyacrylates are toxic and not biodegradable.
At the moment great efforts are being made in order to find new and less environmental harmful substitutive products.
Said products, besides having a good sequestering activity with regard to alkaline-earth metals, must also be biodegradable, not toxic and not expensive.
The citric acid, which is biodegradable and not toxic, is commonly used in the form of sodium salt, but it loses its sequestering power at medium-high temperatures, thus limiting its application range.
Only recently, the study of the application in these fields of alkaline earth metals salified aspartic acid polymers has begun. This product has proved to be very efficient in alkaline earth metal ion complexing, preventing their precipitation in the form of insoluble salts, to be suitable for the above-mentioned application fields and for many others.
The aspartic acid is present in nature in its levorotatory or "l" form, while its racemic form "d,l" is usually obtained by chemical methods.
Both the forms can be used for polyaspartate production, as no significative difference can be noticed from the applicability point of view.
It has to be noticed that while the form "l" can only be obtained by an enzymatic process, starting from ammonium fumarate, the racemic form "d,l" can be obtained much more easily by chemical means.
In the patent literature some processes are already known for the production of polyaspartic acid and the salts thereof, mainly sodium salts.
The first process of interest is the one disclosed in U.S. Pat. No. 4,839,461, by which, through ammonium maleate production from maleic anhydride and ammonia (in a molar ratio varying from 1:1 to 1:5) in aqueous solution, followed by evaporation to dryness of the maleate itself and further heating at a temperature of 120-150.degree. C. and under vacuum, a polymer is obtained with molecular weight comprised between 1000 and 4000, mostly between 1800 and 2000.
The process disclosed in U.S. Pat. No. 4,839,461 is heavily disadvantage from the energy point of view because of the necessity to evaporate a great water amount to pass from the diluted ammonium salt solution to the solid phase in which the product formation reaction is performed.
Moreover, the heating of the solid under static conditions, as those hereinbefore described, presents great problems of temperature uniformity in the mass itself, with consequent formation of a product with not highly homogeneous characteristics and with a broad dispersion of molecular weights. This lack of uniformity causes difficulties in the product's characteristics control as well as a worsening of the sequestering properties. Furthermore it is not sufficiently biodegradable.
Another process is claimed in U.S. Pat. No. 5,057,597 which uses l-aspartic acid as the starting material. The reaction is performed in solid phase by a fluid-bed reactor with reaction temperatures comprised between 180 and 250.degree. C.
The l-aspartic acid, in solid and ground form, is fed into a fluid-bed reactor, which is then heated to about 180.degree. C. and kept at a temperature between 180 and 250.degree. C. for such a period to polymerize the l-aspartic acid and evaporate the reaction water. The final result is a polysuccinimide, which is then hydrolyzed, preferably by means of an alkaline hydrolysis.
The process disclosed in said patent, in addition to the higher complexity of the fluid bed polymerization process, utilizes a very expensive product, because of the high energetic cost for the culture medium concentration and for the obtainment of the dried aspartic acid. Also the polycondensation performed in a fluid bed reactor requires very careful management and does not assure a sufficient conditions uniformity in the whole solid mass. Moreover this product is not biodegradable.
The U.S. Pat. No. 5,219,952 does not use l-aspartic acid but maleic anhydride and ammonia.
In the patent there is claimed the production of polysuccinimide using maleic anhydride and ammonia at high temperatures. The preferred polymerization temperature is between 200-260.degree. C. for a reaction time between 7 and 10 hours, while the preferred molar ratio ammonia-maleic anhydride is from 2 to 3:1. The molecular weight of the final product is from 3200 and 10000.
All the processes currently known both using l and/or d,l aspartic acid, maleic anhydride ammonium salt and/or maleamic acid, lead to the production of polysuccinimide which is then hydrolyzed with alkaline hydroxides to obtain the alkaline salts of the polyaspartic acids.
All the said processes neither can control the finished product molecular weight value, nor can limit the distribution of the molecular weights by themselves. As it is well known in the case of similar products, biodegradability is frequently tied to molecular weights: when the number of the polymer monomeric units exceeds a certain amount, biodegradability lowers until it disappears.
During the thermal treatment the polymer becomes also colored, said color also remaining when from the polymer an aqueous solution is formed to obtain the corresponding alkaline salt. Said coloring is very resistant to any kind of treatment: some improvement is obtained with oxidizing products, the consumption thereof is highly considerable, rendering the decoloring process uneconomical.
The polymers (polysuccinimides) obtained by thermal treatments (on the solid mass in a fluid bed, in a high boiling solvent with or without dehydrating agents) according to the different methods reported in patent literature show completely saturated chains and very different molecular weights, being however that value higher than 1000. ##STR4##